1. Field of the Invention
The present invention relates to a servo control apparatus that performs dual-position feedback control based on the position of a servo motor and the position of a driven load.
2. Description of the Related Art
Servo control for controlling a servo motor that drives a load is classified as semi-closed-loop control, full-closed-loop control, or hybrid control, according to which position information is fed back for the control. In semi-closed-loop control, feedback control is performed by detecting the position of the servo motor. In full-closed-loop control, feedback control is performed by detecting the position of the driven load. However, in hybrid control, feedback control is performed by detecting both the position of the servo motor and the position of the driven load, i.e., dual-position feedback control is performed (for example, refer to patent documents 1 and 2 cited below).
In servo control, when performing a machining operation which is repeated in a periodic fashion, learning control is widely used in order to achieve highly accurate machining by progressively reducing position error close to 0. In this learning control, the position error is compensated by adding the previously applied amount of compensation to the detected position error, and a position control loop is formed based on the thus compensated position error (for example, refer to patent document 3 cited below).
Specifically, patent document 4 cited below discloses an invention in which compensation data for reducing quadrant protrusion is created through learning control and stored, the stored data is then used to perform compensation for the prevention of quadrant protrusion. A quadrant protrusion refers to a protrusion that occurs when the moving direction of the feed shaft is reversed as the machining position changes from one quadrant to another.
When performing the learning control and the quadrant protrusion compensation according to the invention disclosed in patent document 4, if the learning control is performed using the position error of the servo motor in accordance with the semi-closed-loop control, the quadrant protrusion on the servo motor side can be reduced by reducing the position error of the servo motor, but there is a problem in that the quadrant protrusion on the driven load side, i.e., the machine side, cannot be reduced, because the compensation value for reducing the position error on the machine side cannot be obtained.
Conversely, if the learning control is performed using the position error on the machine side in accordance with the full-closed-loop control, the compensation value for reducing the position error on the machine side can be obtained. However, if the position loop gain or speed loop gain is set to a high value, the learning control may not converge due to such factors as the distortion or backlash between the servo motor and the machine, resulting in the problem that the gain cannot be increased. In the case of a machining operation in which the cutting reaction force is small, there will be no problem if the gain is reduced, but in the case of a machining operation that involves a large cutting reaction force, if the gain is reduced, machining accuracy may degrade due to the cutting reaction force.
Patent Document 1: Japanese Examined Patent Publication No. H02-30522
Patent Document 2: Japanese Unexamined Patent Publication No. 2002-297241
Patent Document 3: Japanese Unexamined Patent Publication No. 2006-172149
Patent Document 4: Japanese Unexamined Patent Publication No. 2004-234327